A lithium-ion secondary battery is widely used as a power source for, for example, notebook computers, video cameras, mobile phones, electric cars, etc. As such a lithium-ion secondary battery, a lithium-ion secondary battery having a structure in which a battery main body portion (a main body portion including a positive electrode, a negative electrode, and electrolyte) is surrounded by a case is used. As the casing material (outer packaging material), a packaging material having a structure is publically known in which, for example, an outer layer made of a heat resistant resin film, an aluminum foil layer, and an inner layer made of a thermoplastic resin film are integrally bonded in this order.
In the meantime, in a lithium-ion secondary battery, etc., a gas is easily generated in the battery main body at the time of overcharging and/or overheating, which sometimes causes a case in which a gas is gradually accumulated in the inner space covered by the outer packaging material to thereby increase the internal pressure of the outer packaging material. If this internal pressure increase becomes large, there is a concern that the outer packaging material ruptures to cause scattering of the contained material in the inner portion of the outer packaging material. Therefore, there has been proposed a technology to prevent rupturing of the outer packaging material.
For example, Patent Document 1 describes that, at a portion of a peripheral edge portion of an outer packaging member of a lithium-ion secondary battery, a safety valve for releasing a gas generated in the outer packaging to the outside of the outer packaging is provided.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-265725